The present invention relates to an absorptive circuit element and an absorptive low-pass filter, utilizing frequency selective absorption of a magnetic material, and to a manufacturing method thereof.
In a digital equipment operating at a high clock frequency and in an equipment processing signals with a wide frequency range inside a narrow housing such as a mobile communication equipment, elimination of unnecessary frequency components contained in the signals is important for stabilizing the operation of the equipments.
According to a conventional method for eliminating an unwanted signal, a capacitor with a large capacitance or a general low pass filter was inserted in a source which might produce the unwanted signal to damp unnecessary frequency components in the signal. However, since the large-capacitance capacitor or the general low pass filter was a reactance with a small loss to limit the transmission of the unwanted signal by reflection, the signal reflected by such countermeasure element detoured in other circuits and became new interference sources.
The essential elimination of unnecessary frequency components therefore should not be performed by reflection, but should be performed by absorption.
As for elimination of the unwanted signal by absorption, ferrite beads are currently used. A signal line is covered by the ferrite beads array to configure an inductor with a certain loss so that the unwanted signal is eliminated by the increase in reactance due to frequency and by the magnetic loss. However, since the impedance of the signal line changes depending upon reactance change of the ferrite beads, within a frequency band where the impedance of the ferrite beads is not matching with the output impedance of the unnecessary frequency component source, the unwanted signal is suppressed from passing by reflection. Thus, using of the ferrite beads also cannot be an essential countermeasure. Furthermore, absorption of the ferrite beads rapidly decreases at 1 GHz or more due to performance restriction of the material. Hence, the ferrite beads are insufficient for a countermeasure against the unwanted signal in a recent mobile communication equipment and a high-speed data bus circuit.
In order to solve these problems, the present applicant has proposed an absorptive low-pass filter element exhibiting a small reflection and a large absorption in an unwanted signal processing frequency band (Japanese patent unexamined publication No.08204486A).
FIG. 1 is a partially cutaway oblique view schematically illustrating a structure of this conventional absorptive low-pass filter element.
In the figure, reference numeral 10 denotes a magnetic material core provided in a center section and formed by ferrite or fine powder of pure iron bound with a resin, 11 a conductor (inner conductor) helically wound around the magnetic core 10, 12 a magnetic material provided outside the inner conductor 11 and formed by binding fine pure iron powder with a resin, and 13 an outer conductor formed on the surface of the magnetic material 12 to be made conductive, respectively.
By electrically dividing this outer conductor 13 as shown in FIG. 1 into three sections, by applying a signal across the two conductor sections in both end surfaces (input and output terminals) 13a and 13b and by grounding the central conductor section (ground conductor) 13c, the inner conductor 11 and the outer conductor 13 configure a transmission line with a certain loss. Since this line is distributed-constant structure, a characteristic impedance of the filter element is determined by its line structure and by real parts of a permeability and a dielectric constant of the magnetic material, and loss is determined by the magnetic loss of the magnetic material. If the characteristic impedance is set at a value near to a drive impedance, it is possible to absorb the unwanted signal energy by the loss in the filter element while suppressing reflection from the filter element as much as possible.
When such low pass filter is terminated, as shown in FIG. 2, a reflection coefficient (reflection loss) S11 of an input terminal is xe2x88x9210 dB or less over the entire frequency range, but a transmission coefficient (transmission loss) S21 exhibits a low-pass or high-cut filtering characteristic. If this filter element is inserted into a high frequency circuit, signals at or below a cutoff frequency will pass as it is, but signals over the cutoff frequency will be absorbed inside the element and will not be transmitted resulting that it is possible to eliminate the signals over the cutoff frequency from the high frequency circuit. If an element other than a terminator is connected to the output side of this filter element however, its impedance is reflected to the input side in a low-pass frequency region of the transmission coefficient S21 (Japanese patent unexamined publication No.08204486A).
It is difficult to precisely find a characteristic impedance Z0 of such compact filter element. Nevertheless, by modeling the line structure on a microstrip line, it is possible to roughly calculate the characteristic impedance Z0 from formula (1) with a width W0 of the inner conductor, a thickness h of the magnetic material, and a relative permeability xcexcr and a relative dielectric constant ∈r of the magnetic material.                               Z          0                =                                                            μ                r                                            ϵ                r                                              ⁢          601          ⁢                      n            ⁡                          (                                                                    8                    ⁢                    h                                                        W                    0                                                  -                0.358                +                                  1                                                                                    0.931                        ⁢                        h                                                                    W                        0                                                              +                    0.736                                                              )                                                          (        1        )            
Now, let W0 be 0.15 mm, and h be 0.2 mm. Because xcexcr=9 and ∈r=90 in the magnetic material containing 85 wt % of fine pure iron powder, the characteristic impedance of this filter element becomes Z0=45.2 xcexa9 from formula (1).
However, if such filter element is connected to a drive element with a high impedance, an unwanted signal suppression effect becomes insufficient due to the reflection caused by impedance-mismatching. In addition, if contents of the fine pure iron powder in the magnetic material 12 is increased to enhance the magnetic loss (absorption amount), the input impedance of the element remarkably drops because of the increase of an effective dielectric constant resulting the unwanted signal suppression effect to become further insufficient due to the mismatching in impedance.
If the contents of iron powder is made at 90 wt % or more in the magnetic material 12 in order to increase the magnetic loss, breakdown may occur because of contact between particles of the iron powder and a short-circuit with the ground conductor may occur. This is called a leakage current failure that should be avoided in any electronic component.
It is therefore an object of the present invention to provide an absorptive circuit element and an absorptive low-pass filter, in which input impedance can be determined independently of an absorption characteristic, and a manufacturing method thereof.
According to the present invention, an absorptive circuit element includes a core body made of non-conductive material, an inner conductor formed by winding a conductive wire around the core body with a gap provided between adjacent turns, a magnetic material surrounding outside of the inner conductor, the magnetic material being made of composite material containing ferromagnetic fine metal powder and insulating resin, a dielectric surrounding outside of the magnetic material, and an outer conductor formed on a surface of the dielectric. An absorptive low-pass filter according to the present invention is provided with this absorptive circuit element.
An absorptive circuit element according to the present invention processes unnecessary frequency components in a signal by absorption, not by reflection. Owing to this, the circuit element of the present invention is remarkably useful for elimination of an interfering wave inside a computer with a high frequency clock and a mobile communication equipment which processes a signal with a wide frequency range in a narrow housing.
Particularly, according to the present invention, in the absorptive circuit element exhibiting a large magnetic loss, an effective dielectric constant of the magnetic material between the inner conductor and the outer conductor or ground conductor is greatly dropped without decreasing its effective permeability by forming the dielectric outside the magnetic material so as to prevent decrease in the input impedance and any leakage current failure from occurring. Thus, the difference between the drive impedance of an unwanted signal source and the input impedance of the absorptive circuit element is decreased to reduce the reflection of the unwanted signal while keeping the attenuation of the unwanted signal, so that the unwanted signal can be effectively eliminated.
In other words, according to the present invention, the input impedance is controlled independently of the absorption inside the absorptive circuit element by dropping the effective dielectric constant between the inner conductor and the outer conductor or ground conductor with suppressing the degradation of the permeability through sandwiching the dielectric between the magnetic material surrounding the inner conductor and the ground conductor. As shown in the formula (1), a line impedance of the absorptive circuit element can be controlled by the permeability and the dielectric constant of a member for supporting the line.
Hereinafter, changes in the permeability and the dielectric constant in case that a dielectric is inserted between a magnetic material and a ground conductor are theoretically calculated.
If a transmission mode in the line is a TEM mode, the law of mapping is established. Hence, the following calculation uses a coaxial line model to obtain an exact solution.
Let the structure of a coaxial line is as shown in FIG. 3, and let a is an inner diameter, b an outer diameter, k an outer diameter of a magnetic material, xcexc1 a relative permeability of the magnetic material, ∈1 a relative dielectric constant of the magnetic material, xcexc2 a relative permeability of a dielectric sandwiched between the magnetic material and a ground conductor, ∈2 a relative dielectric constant of the dielectric sandwiched between the magnetic material and the ground conductor, xcexc0 a space permeability, and ∈0 a space dielectric constant.
In this coaxial line, an inductance per unit length Lxe2x80x2 is given in formula (2):                               L          xe2x80x2                =                                                            ∫                a                k                            ⁢                                                μ                  0                                ⁢                                  μ                  1                                ⁢                                  xe2x80x83                                ⁢                                                      ⅆ                    r                                                        2                    ⁢                    π                    ⁢                                          xe2x80x83                                        ⁢                    r                                                                        +                                          ∫                k                b                            ⁢                                                μ                  0                                ⁢                                  μ                  2                                ⁢                                  xe2x80x83                                ⁢                                                      ⅆ                    r                                                        2                    ⁢                    π                    ⁢                                          xe2x80x83                                        ⁢                    r                                                                                =                                                    μ                0                            ⁢                              μ                1                            ⁢              1              ⁢              n              ⁢                              k                a                                      +                                          μ                0                            ⁢                              μ                2                            ⁢              1              ⁢              n              ⁢                                                b                  k                                .                                                                        (        2        )            
Now, let L be:   L  =                    ∫        a        b            ⁢                        μ          0                ⁢                  μ          1                ⁢                  xe2x80x83                ⁢                              ⅆ            r                                2            ⁢            π            ⁢                          xe2x80x83                        ⁢            r                                =                            μ          0                ⁢                  μ          1                ⁢        n        ⁢                  b          a                    ≡                        μ          1                ⁢                  L          0                    
then, formula (2) is transformed as follows:                               L          xe2x80x2                =                                                            μ                1                            ⁡                              [                                  1                  +                                                            (                                                                                                    μ                            2                                                                                μ                            1                                                                          -                        1                                            )                                        ⁢                                                                  1                        ⁢                        n                        ⁢                                                  b                          k                                                                                            1                        ⁢                        n                        ⁢                                                  b                          a                                                                                                                    ]                                      ⁢                          L              0                                ≡                                    μ              eff                        ⁢                          L              0                                                          (        3        )            
and an effective permeability xcexceff becomes as follows:                                           μ            eff                    =                                    μ              1                        ⁡                          [                              1                +                                                      (                                                                                            μ                          2                                                                          μ                          1                                                                    -                      1                                        )                                    ⁢                  χ                                            ]                                      ⁢                  
                ⁢                  where          ,                      χ            ≡                          1              ⁢              n              ⁢                                                b                  k                                /                1                            ⁢              n              ⁢                                                b                  a                                .                                                                        (        4        )            
According to this formula, it can be seen that the effective permeability decreases depending upon a ratio of the inside and outside permeability of the coaxial line. On the other hand, capacitance per unit length of the coaxial line, Cxe2x80x2 is calculated from a model configured by connecting thin coaxial lines in series:                                                                         1                                  C                  xe2x80x2                                            =                                                                    ∫                    a                    k                                    ⁢                                      xe2x80x83                                    ⁢                                                            ⅆ                      r                                                                                      ϵ                        0                                            ⁢                                              ϵ                        1                                            ⁢                      2                      ⁢                      π                      ⁢                                              xe2x80x83                                            ⁢                      r                                                                      +                                                      ∫                    k                    b                                    ⁢                                      xe2x80x83                                    ⁢                                                            ⅆ                      r                                                                                      ϵ                        0                                            ⁢                                              ϵ                        2                                            ⁢                      2                      ⁢                      π                      ⁢                                              xe2x80x83                                            ⁢                      r                                                                                                                                              =                                                                    1                                          2                      ⁢                                              πϵ                        0                                            ⁢                                              ϵ                        1                                                                              ⁢                  1                  ⁢                  n                  ⁢                                      k                    a                                                  +                                                      1                                          2                      ⁢                      π                      ⁢                                              xe2x80x83                                            ⁢                                              ϵ                        0                                            ⁢                                              ϵ                        2                                                                              ⁢                  1                  ⁢                  n                  ⁢                                                            b                      k                                        .                                                                                                          (        5        )            
From formula (5):                               C          xe2x80x2                ⁢                  xe2x80x83                =                  xe2x80x83                ⁢                                            ϵ              1                        ⁢                          xe2x80x83                        ⁢                          1                              1                ⁢                                  xe2x80x83                                +                                  xe2x80x83                                ⁢                                                      (                                                                                            ϵ                          1                                                                          ϵ                          2                                                                    ⁢                                              xe2x80x83                                            -                                              xe2x80x83                                            ⁢                      1                                        )                                    ⁢                                      xe2x80x83                                    ⁢                                                                                    xe2x80x83                                            ⁢                                              1                        ⁢                                                  xe2x80x83                                                ⁢                        n                        ⁢                                                  b                          k                                                                                                            1                      ⁢                                              xe2x80x83                                            ⁢                      n                      ⁢                                              xe2x80x83                                            ⁢                                              b                        a                                                                                                                  ⁢                          xe2x80x83                        ⁢                          C              0                                ⁢                      xe2x80x83                    ≡                      xe2x80x83                    ⁢                                    ϵ              eff                        ⁢                          xe2x80x83                        ⁢                          C              0                                                          (        6        )            
Similarly to the case of inductance, let C0 be:             C      0        ≡                  2        ⁢                  πϵ          0                            1        ⁢        n        ⁢                  b          a                      ,
an effective dielectric constant ∈eff becomes as follows:                               ε          eff                =                              ε            1                    ⁢                                    1                                                (                                                                                    ε                        1                                                                    ε                        2                                                              -                    1                                    )                                ⁢                χ                                      .                                              (        7        )            
Assuming that the dielectric located at the outside is thin, the effective dielectric constant can be approximated as follows:       ϵ    eff    ≅                    ϵ        1            ⁡              [                  1          -                                    (                                                                    ϵ                    1                                                        ϵ                    2                                                  -                1                            )                        ⁢            χ                          ]              .  
According to this formula, it can be seen that, although the effective dielectric constant decreases with the thickness of the dielectric, contribution of the dielectric constant of material located in the inside is large differently from the case of permeability.
As used in the former calculation, using xcexc1=9 and ∈1=90, and selecting a paraelectric material such as plastic as the dielectric, sandwiched between the magnetic material and the ground conductor, it is assumed that xcexc2=1, ∈2=2.5. Changes of the effective permeability xcexceff and effective dielectric constant ∈eff are obtained as follows by inserting the dielectric through substituting this data in formula (7),
xcexceff=9(1xe2x88x920.89x)xe2x80x83xe2x80x83(8)
xe2x80x83∈eff≅90(1xe2x88x9235x)xe2x80x83xe2x80x83(9).
As is apparent from formulas (8) and (9), the effective dielectric constant ∈eff decreases at 39.3 times the speed of the effective permeability xcexceff. Therefore, by inserting a paraelectric material with a small dielectric constant between the magnetic material and the ground conductor, it is possible to decrease the effective dielectric constant while suppressing a change of the effective permeability. In addition, even if the insulation of the magnetic material layer is destroyed by the increase of the quantity of iron powder, it becomes possible to prevent current leakage by inserting the dielectric to provide a structure having large freedom in characteristics design.
It is preferred that the outer conductor is located along an axial direction of the absorptive circuit element, and consists of three portions electrically separated from each other.
It is also preferred that the three portions of the outer conductor are electrically separated at locations each xc2xc of axial length of the absorptive circuit element distance from each end of the absorptive circuit element in the axial direction.
Preferably, both end portions of the three portions of the outer conductor are electrically connected to both ends of the inner conductor, respectively.
Also, preferably, both end portions of the three portions of the outer conductor function as input and output terminal of the absorptive circuit element, respectively.
Furthermore, preferably, a central portion of the three portions of the outer conductor functions as a ground conductor of the inner conductor.
It is preferred that the magnetic material is a magnetic material exhibiting a frequency selective absorption characteristic, particularly that the magnetic material is a magnetic material exhibiting an absorption characteristic in a high frequency region.
It is also preferred that a width of the inner conductor, a thickness of the magnetic material, a permeability of the magnetic material and a dielectric constant of the magnetic material are determined so that an input impedance in an absorption band does not depend on a frequency in the high frequency region.
It is preferred that a thickness of the dielectric is set so that a reflection characteristic does not depend on an absorption characteristic.
It is further preferred that the dielectric is colored so as to identify the absorptive circuit element.
It is preferred that the core body is made of a ferrite magnetic material, a paraelectric material or a high resistance material.
According to the present invention, also a manufacturing method of an absorptive low-pass filter includes a step of forming an inner conductor by winding a conductive wire around an outer peripheral surface of a core body made of non-conductive material with a gap provided between adjacent turns, a step of surrounding an outer peripheral surface of the core body, around which the inner conductor is formed, with a magnetic material made of composite material of ferromagnetic fine metal powder and insulating resin, a step of forming a bar structure with surrounding an outer peripheral surface of the magnetic material with a dielectric, a step of forming a plurality of separated elemental pieces by cutting the bar structure along planes orthogonal to an axis of the bar structure, and a step of forming an outer conductor on a surface of each separated elemental piece.
It is preferred that the outer conductors are formed by forming a conductive layer over all surfaces of each separated elemental piece and thereafter by electrically separating the conductive layer into three portions located along an axial direction of the core body.
It is also preferred that the conductive layer and both ends of the inner conductor are electrically connected with each other when the conductive layer is formed over all surfaces of each separated elemental piece.